According to the release notes,
Tor Browser 4.5.2 is here to fix a Logjam attack, and updates the Tor
(The Onion Router) anonymity network software to version 0.2.6.9, the
OpenSSL library to version 1.0.1n, the HTTPS-Everywhere and NoScript
add-ons to version 5.0.5 and 2.6.9.26 respectively, and brings Torbutton
to version 1.9.2.6.

"Tor Browser 4.5.2 provides a fix for the Logjam
attack (https://weakdh.org/) and updates a number of Tor Browser
components," says Mike Perry. "Moreover, it fixes a possible crash on
Linux and avoids breaking the Add-ons page if Torbutton is disabled."